1. Field of the Invention
The present invention is directed to an image forming apparatus, such as a copying machine and a printer, in which an electrophotographic system is utilized and a development device used in the image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus of the related art, charged toner is brought close to an image bearing member, and the toner is caused to adhere electrostatically to an electrostatic latent image on the image bearing member, thereby performing development to form an image. A two-component developer mainly containing non-magnetic toner and a magnetic carrier is widely used as a developer.
A development bias that moves the developer to the image bearing member includes an AC development in which an AC voltage is superimposed on a DC voltage and a DC development in which only the DC voltage is utilized. While the AC development is excellent for image quality, the DC development has advantages in a simple configuration and low cost.
For example, a bias in which the rectangular-wave AC voltage of about Vp-p=2 kV is superimposed on the DC voltage of about 200 V is used in a two-component AC development method in which the two-component developer is used. A DC component of the development bias is applied in order to send the toner in the developer on a surface of a developer bearing member toward the image bearing member. On the other hand, an AC component has an effect that separates the toner and carrier, which are charged in opposite-polarities, in the developer by instantaneously applying a large electric field.
The development is performed only by the DC voltage in a two-component DC development method in which the two-component developer is used. Because the DC voltage of the two-component DC development method is smaller than the AC voltage of the two-component AC development method, the separation action of the development bias on the toner and the carrier is small. Therefore, a small number of toner particles can move under the development bias, and the amount of toner developed in the image bearing member is inevitably decreased to hardly obtain desired image density.
For example, a method for increasing a toner concentration in the developer, a method for enhancing a developer conveying speed of the developer bearing member, and a method for increasing the development bias are proposed in order to improve the image density. However, when the toner concentration is increased in the developer, what is called a fog in which the toner adheres to a non-image region on the image bearing member is easy to generate. When the developer conveying speed is enhanced, it is well known that degradation of the developer is accelerated.
When the development bias is increased, because a force acting on the toner is increased, a separation characteristic of the toner and the carrier can be improved to increase the image density. However, because a probability that the carrier adheres to an image region on the image bearing member is significantly increased, there is a limitation to the increase in image density by increasing the development bias.
In Japanese Patent Laid-Open No. 4-70874, an excitation electrode is provided in front of the development region in order to attract the toner in the two-component developer. Therefore, when the developer reaches the development region through an excitation electrode position, the toner in the developer is attracted onto the excitation electrode side to enhance the toner concentration of the developer that comes into contact with a photosensitive body.
However, in the technology of Japanese Patent Application Laid-Open No. 4-70874, the force moving the toner in the developer remains weak, and the electrode has only a small effect.